CVE-2026-39410

A discrepancy between browser cookie parsing and parse() handling allows cookie prefix protections to be bypassed.

Cookie names that are treated as distinct by the browser may be normalized to the same key by parse(), allowing attacker-controlled cookies to override legitimate ones.

Details

Browsers follow RFC 6265bis and only trim SP (0x20) and HTAB (0x09) from cookie names. Other characters, such as the non-breaking space (U+00A0), are preserved as part of the cookie name.

For example, the browser treats the following cookies as distinct:

"dummy-cookie"
"\u00a0dummy-cookie"

However, parse() previously used JavaScript's trim(), which removes a broader set of characters including U+00A0. As a result, both names are normalized to:

"dummy-cookie"

This mismatch allows attacker-controlled cookies with a U+00A0 prefix to shadow or override legitimate cookies when accessed via getCookie().

An attacker who can set cookies (e.g., via a man-in-the-middle on a non-secure page or other injection vector) can bypass cookie prefix protections and override sensitive cookies.

This may lead to:

• Bypassing __Secure- and __Host- prefix protections
• Overriding cookies that rely on the Secure attribute
• Session fixation or session hijacking depending on application usage

This issue affects applications that rely on getCookie() for security-sensitive cookie handling.

The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.

CVE-2026-39410 has a CVSS score of 4.8 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.12.12); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-39410? CVE-2026-39410 is a medium-severity improper input validation vulnerability in hono (npm), affecting versions < 4.12.12. It is fixed in 4.12.12. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
2. How severe is CVE-2026-39410? CVE-2026-39410 has a CVSS score of 4.8 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of hono are affected by CVE-2026-39410? hono (npm) versions < 4.12.12 is affected.
4. Is there a fix for CVE-2026-39410? Yes. CVE-2026-39410 is fixed in 4.12.12. Upgrade to this version or later.
5. Is CVE-2026-39410 exploitable, and should I be worried? Whether CVE-2026-39410 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-39410 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-39410? Upgrade hono to 4.12.12 or later.